Hail occurs in the summer when strong thunderstorm updrafts carry raindrops high into the atmosphere, where temperatures are below freezing, according to the Atmospheric Sciences Department of Texas A&M University. This causes the water to freeze into pellets of ice, forming hailstones. These pellets may rise and fall multiple times in the storm before falling to the ground, resulting in hailstones of varying sizes.
Hail requires a very specific set of thermal circumstances. The lower levels of a thunderstorm must have air temperatures well above freezing to allow water to fall as liquid droplets, while higher levels of the storm must be well below freezing to allow the water droplets to freeze quickly. The storm also needs strong updrafts between these two levels to carry the liquid water high enough to freeze. Provided that the ice pellets are large enough to avoid melting on the way down to the ground, the resulting precipitation will fall as hail.
Many of the same conditions that make a storm suitable for hail also make it suitable for the formation of tornadoes, which is why these meteorological conditions sometimes occur simultaneously. The heaviest hailstone on record weighed 2.2 pounds, and the largest hailstone measured 8 inches across.